Capacitative panel touchscreens have been used as user input devices for many years, yet their usage is still increasing with the more widespread adoption of touchscreen computing devices. Further, the recent development of capacitative touchscreens that can detect and recognize multiple touches, so called multi-touch sensing, has further increased the usage of such touchscreens.
It is desirable to sense variations in the capacitances of and between rows and columns of a grid of conductors on a panel, for example to allow multi-touch interaction with a computer program.
Methods suitable for sensing multi-touch on small panels are known, but they scale poorly to large panels because: the length of time required for a scan is fixed (by the voltage, size of the capacitive elements and other physical factors) and therefore the length of time available for measuring a given capacitance decreases as panels grow larger; because larger panels are more susceptible to interference; because coupling between adjacent conductors is stronger on larger panels; and because power consumption increases with larger panels.
Existing methods attempt to compensate for reduced measurement time by driving the panel with higher voltages, but this causes practical circuit difficulties, increases electromagnetic interference from the panel, and is still sensitive to impulse disturbances. Operating at higher test-signal frequencies also increases electromagnetic interference and in addition causes interaction between rows of the grid.